Imagine yourself sitting in a roller coaster, waiting for the operator to press the button or pull the lever to start the ride. In technical terms, you are waiting for him to actuate a DCV (Direction control valve). Yes, new tech roller coasters work on hydraulic systems to gain quick acceleration. A very good example is the Top Hill Dragster which gains 120Mph in 4 seconds.
In a hydraulic system, the motion of the actuator is controlled by a small element known as the “direction control valve (DCV)”. The purpose of this article is to make you aware of the parameters which play a major role in the selection of the DCV and can serve as your best friend while making a purchase.
The DCVs are classified by
- Number of ports
- Number of positions
- Method of actuation
- Type of construction
Number of ports on a DCV
The number of ports depends upon the application for which the DCV is being used. Every DCV has at least 2 ports – the pump port and tank port. A single acting hydraulic cylinder has only 1 inlet port and hence it needs only one connection from a DCV. So in all single acting cylinder applications a 3 port DCV will do the job. Similarly a double acting cylinder will demand 2 ports connected to the DCV and hence, one should buy a 4 port DCV.
Number of positions on a DCV
The number of positions associated with a DCV depends upon at which points the cylinder needs to stop. If you want the cylinder to stop at either end only, a 2 position DCV will serve the purpose. To stop the cylinder at any intermediate position, one more neutral position is required as shown in the figure below.
The neutral position is a very important part of the criteria when selecting a DCV – hydraulic engineers will invest considerable time to ensure they select the best DCV for the job.
Method of actuation
DCVs can be actuated by several means. Some of the most commonly used ways are covered below:-
Manually operated DCV
A DCV can be actuated by manually shifting the lever or foot pedal. This reduces the cost of each DCV drastically and is mostly used for intermittent application. The force required to actuate a DCV increases with the pressure and hence it is not advisable to use this system above 100-150bar.
Solenoid operated DCV
Magnetic force can be generated by passing a current through an electric coil. This force will push the armature into the coil which in turn actuates the DCV. Solenoids can be controlled by logic controllers and from a remote location hence it is used by all of the major industries. However, solenoids cannot generate high forces and for high pressure hydraulic circuits they are generally used with small pilot DCVs which in turn will actuate a larger DCV.
Type of construction
The flow rate is controlled by the upward/downward motion of the poppet. Poppet is connected with a sphere/disk at the bottom and as the poppet moves upward, the gap between poppet and seat increases and hence, flow rate increases. Actuation force depends upon the system pressure and hence actuation is controlled by solenoids.
Pros: Cheap, no leakage, long life, any flow rate between min to max value is possible.
Cons: Actuation force is proportional to pressure hence they are used for low pressure systems.
Spool valves are constructed with a spool moving horizontally within the valve. It can deal with a flow rate of 150ltr/min and up to 350bar pressure.
Pros: Balanced forces in the valve. Can allow very high flow rate.
Cons: Leakage due to the clearance between spool and valve body (3 to 15 micron). Costly due to precise manufacturing of the internal bore and spool.